(1) Field of the Invention
The present invention relates to a subscriber system (access system) transmission apparatus with a line concentrating capability, and more particularly to a subscriber system transmission apparatus with a line concentrating capability, which comprises one or more remote terminals connected to subscriber""s terminals and a single central terminal connected to an exchange.
(2) Description of the Related Art
The subscriber system transmission apparatus with the line concentrating capability according to the present invention is applicable to PDH (Plesiochronous Digital Hierarchy) or SDH (Synchronous Digital Hierarchy), and is governed by the V5.2 protocol according to the ITU-T (International Telecommunication Unionxe2x80x94Telecommunication Sector).
FIG. 8 of the accompanying drawings shows in block form a conventional point-to-point subscriber system transmission apparatus with a line concentrating capability. As shown in FIG. 8, the subscriber system transmission apparatus comprises a RT (Remote Terminal) transmission device 110 with a line concentrating capability and a CT (Central Terminal) transmission device 120 with no line concentrating capability, the RT transmission device 110 and the CT transmission device 120 being connected to each other. A plurality of subscriber""s terminals 100, each in the form of a telephone set, are connected to the RT transmission device 110, and an exchange 130 is connected to the CT transmission device 120. The RT transmission device 110 and the CT transmission device 120 may be installed at locations that are several kilometers spaced from each other.
The RT transmission device 110 comprises a subscriber circuit (CH) 111 for converting an analog signal to a digital signal, a tributary bus (Bus) 112, a line concentrator connector (TSA) 113 for assigning time slots, a trunk bus (Bus) 114, a terminal unit (OPT) 115 for converting an electric signal to an optical signal, and a line concentration controller (V5MC) 116 for controlling the line concentrator connector 113 to effect a line concentrating process. The line concentration controller 116 and the line concentrator connector 113 jointly effect a line concentrating process which assigns a call to an idle channel in a trunk transmission line each time the call is requested from a subscriber""s terminal 100. Even if the trunk transmission line has a smaller number of channels than the number of subscriber""s terminals 100, the trunk transmission line can connect calls from subscriber""s terminals 100.
The CT transmission device 120 comprises a terminal unit (OPT) 121 for converting an optical signal to an electric signal, a trunk bus (Bus) 122, a time slot assignment unit (TSA) 123 for assigning time slots, a tributary bus (Bus) 124, and a terminal unit (Trib Card) 125 for interfacing signals according to the V5.2 protocol. The CT transmission device 120 has no line concentrating capability.
To provide against a fault of the line concentrating capability, the line concentration controller 116 and the line concentrator connector 113 have a redundant structure.
If the subscriber system transmission apparatus with a line concentrating capability is of the point-to-multipoint call processing type, then the redundant structure is so complex that the circuit arrangement which is required contains superfluous circuit elements.
FIG. 9 of the accompanying drawings shows in block form a conventional point-to-multipoint subscriber system transmission apparatus with a line concentrating capability. As shown in FIG. 9, the point-to-multipoint subscriber system transmission apparatus has n RT transmission devices (RT#1-RT#n) 210-213 connected to a single CT transmission device (CT) 220. Each of the RT transmission devices 210-213 has an internal structure which is identical to the internal structure of the RT transmission device 110 shown in FIG. 8. The CT transmission device 220 has an internal structure which is the same as the internal structure of the CT transmission device 120 shown in FIG. 8 except that the transmission device 220 has a plurality of terminal units (OPT) 221a-221d, each identical of the optical terminal 121 shown in FIG. 8, which are connected respectively to the RT transmission devices 210-213. A plurality of sets of subscriber""s terminals 200, 201, 202, 203, each in the form of a telephone set, are connected to the RT transmission devices 210-213, respectively, and an exchange 230 is connected to the CT transmission device 220.
If the point-to-multipoint subscriber system transmission apparatus has a redundant structure associated with the line concentrating capability for protection against a fault of the line concentrating capability, then each of the RT transmission devices 210-213 needs to have two line concentration controllers and two time slot assignment units. As a result, the point-to-multipoint subscriber system transmission apparatus is necessarily large in system arrangement. Accordingly, it is desirable to simplify the system arrangement of the point-to-multipoint subscriber system transmission apparatus and efficiently utilize the circuit arrangement thereof.
In the event of a fault in either one of the channels in the trunk transmission line of the RT transmission device 110 shown in FIG. 8, the block loss probability increases to the extent that it will be difficult for telephone calls to be connected. The block loss probability is a probability that a call connection will be rejected because the trunk transmission line has no available idle channel for a call request from a subscriber""s terminal 100. This problem also occurs in the point-to-multipoint subscriber system transmission apparatus.
Furthermore, if an increased number of calls are made by subscriber""s terminals 100 in the RT transmission device 110 shown in FIG. 8, then the block loss probability also increases, making it difficult for those calls to be connected. The point-to-multipoint subscriber system transmission apparatus also suffers this drawback.
It is therefore an object of the present invention to provide a subscriber system transmission apparatus with a line concentrating capability which is performed by a relatively simple efficient redundant structure for increased overall system reliability.
To achieve the above object, there is provided in accordance with the present invention a subscriber system transmission apparatus with a line concentrating capability, having a remote terminal connected to a subscriber""s terminal and a central terminal connected to an exchange. The subscriber system transmission apparatus has remote terminal line concentrating and connecting means disposed in the remote terminal, for effecting a line concentrating process on a call from the subscriber""s terminal to assign the call to a trunk transmission line, bypassing means disposed in the remote terminal, for bypassing the remote terminal line concentrating and connecting means to connect a call from the subscriber""s terminal directly to the trunk transmission line, mode selecting means disposed in the remote terminal, for selecting a mode of operation of the remote terminal line concentrating and connecting means when the remote terminal line concentrating and connecting means operates normally and selecting a mode of operation of the bypassing means when the remote terminal line concentrating and connecting means suffers a fault, central terminal line concentrating and connecting means disposed in the central terminal, for effecting a line concentrating process on a call from the remote terminal to assign the call to an exchange transmission line, and selecting means for sending a call from the remote terminal to the exchange transmission line when the remote terminal line concentrating and connecting means operates normally and operating the central terminal line concentrating and connecting means when the remote terminal line concentrating and connecting means suffers a fault.